Thermoplastic information recording utilizing electrets



fiept. 27, 1966 J. GAYNOR 3,276,031

THERMOPLASTIC INFORMATION RECORDING UTILIZING ELECTRETS Filed Jan. 14. 1963 [rm/enter His flttorney //%i Jose v}; 6

United States Patent 3,276,031 THERMOPLASTIC INFORMATION RECORDING UTILIZING ELECTRETS Joseph Gaynor, Schenectady, NY assignor to General Electric Company, a corporation of New York Filed Jan. 14, 1963, Ser. No. 251,370 9 Claims. (Cl. 346-74) This invention pertains generallyv to the recording of information by selectively neutralizing an electrically polarized dielectric medium and converting the residual charge pattern into minute physical deformations on the surface of the medium. More particularly, this invention pertains to a recording system which selectively depolarizes a deformable electret member by energy means to establish a latent image therein and converts the latent image to a relief pattern of deformations on the surface of the medium.

Various information storage processes have been developed to produce information-bearing deformations on the surface of a deformable dielectric member. In a number of these processes, the information storage member is uniformly charged electrostatically and thereafter selectively discharged in the desired information storage pattern so that heating of the selectively discharged member to a softened condition allows the individual electrostatic forces to deform the surface thereby producing a visible relief image. Unfortunately, a surface-charged member is susceptible to unintentional discharge from a variety of conditions including accidental contact, ambient relative humidity above normal, and even ordinary discharge to ground. Additionally, selective discharge of a surfacecharged member is dependent upon the voltage gradient existing between the charged surfaceand its ground plane which is also subject to variation with changing ambient conditions. Since the potential gradient is often established across more than one layer of the recording member, the resolution capability of information storage should be less than attainable where all electrical stress is contained within a single layer. The buried-charge of an electret layer is not subject to discharge as above recited and exerts electrical stress entirely within a single layer. An electret layer offers further advantages in a recording process of the type described wherein it may be desirable to break-up an image projected onto the recording member to provide shading gradations in the final recording if photographic subjects are being recorded. No further means are required to break-up a projected image in the present recording process if the electret charge pattern itself is arranged to dissect the impinging radiation. More particularly, if the charge pattern making up the original electret layer is oriented in a pattern of close-spaced parallel line charges which can be accomplished by known technique, subsequent selective discharge of the pattern responsive to a projected image produces a fragmented charge pattern exhibiting the desired shade tones. Utilization of a charged electret layer for recording in the general manner described eliminates any necessity for electrosta-tically precharging the recording medium during the recording process. Since the electret layer undergoes only reversible physical change during the recording process, as will be described in greater detail hereinafter in the specification, a reusable recording member is provided after erasure of any previously stored information.

It is an important object of the invention, therefore, to provide a system for storing information as permanent physical deformations 0n the surface of a suitable recording member by a method which requires no electrostatic prechargin-g of the storage member.

It is another important object of the invention to pro- 3,276,031 Patented Sept, 27, 196 6 vide a system for recording information in the form of permanent physical deformations on the surface of a supported electret layer by means of selectively neutralizing the electret charge and heating the selectively neutralized medium to form the deformations.

Still another important object of the invention is to provide a system for producing a visible image on a supported electret layer which can be erasedto permit re cording of additional information on the number.

Another important object of the invention is to provide an information storage system whereby information is record-ed solely by exposing a supported electret layer to an electrical charge pattern of the information being re corded and heating the exposed electret layer to produce a surface deformation image of residual charge pattern thereon.

A still further important object of the invention is to provide a system for recording the information-bearing deformation image on the surface of a supported electret layer which is :also photoconductive by selective neutralizing the electret charge pattern with photoradiati-on and heating the exposed electret layer to produce the deformation image.

These and other important objects and advantages of the invention will be apparent from the following description taken in connection with the accompanying drawings in which:

FIGURE 1 is a cross-sectional view of one information storage member of the invention containing informationbearing deformations;

FIGURE 2 depicts schematically a recording system for producing a deformation image on the surface of a suitable storage member in accordance with the principles of the invention;

FIGURE 3 represents still another recording system of the invention utilizing a different information storage member; and

FIGURE 4 is a perspective view, partially in cross section, illustrating an information storage member of the invention in the form of an endless flexible tape.

Briefly, the improved recording process of the invention comprises selectively neutralizing a supported deformable electret layer by energy means and heating the selectively neutralized electret layer to a softened condition whereupon deformations are produced on the surface of the layer corresponding in location to remaining areas of polarization in the layer. The deformations are formed on the surface of the softened electret layer and comprise crests and depressions in the surface formed by movement of the softened material responsive to the forces generated by the residual polarization. Cooling of the softened material to a normally solid condition preserves the deformations permanently unless purposely removed by further heating to permit ultimate leveling of the liquified material.

Neutralization of the electret charge by energy means during the recording process is produced in all cases by charge cancellation in those portions of the electret layer receiving the energy which results in selective depolarization of the electret material. The energy means appropriate for the neutralization is particular to the depolarization characteristics of the electret material being employed and may comprise a variety of energy forms ineluding electron discharge sources, ionizing radiation, and photoradiation. -If conventional deformable electret materials are employed which may generally be characterized as volume-polarized electrically insulative materials, any energy source capable of providing electrons in the electret layer may be employed for the selective charge cancellation. Positively charged portions of the polarized molecules in the electret layer receiving the electrons face of the layer.

will be neutralized and capable of random movement in the subsequently softened material. On the other hand, the unalfected still-polarized molecules in a softened electret layer remain subject to the original polarization stress so as to provide the necessary mechanical forces required for production of the characteristic deformation pattern previously recited. If the electret material is also photoconductive, the selective depolarization may be produced solely by exposure of the electret layer to photoradiation in the desired pattern to be recorded. While the mechanism of selective depolarization is believed to differ in the medium from that described for the more conventional electret materials, the charge pattern of the photoconductive electret layer is cancelled in those portions receiving the photoradiation. The irradiated photoconductive molecules provide the neutralizing charges to effect charge cancellation in the surrounding electret molecules.

Having described the invention generally, it may be practiced in its preferred embodiments as depicted in the accompanying drawings. As used hereinafter in the specification and claims to describe the novel recording memher and process of the invention, the term electret is used in its generally accepted sense to define a permanently electrified substance under ordinary conditions having electrical charges of opposite sign at each major surface of the layer. Useful electret layers for recording are characterized further as volume polarized so as to have a number of subsurface charges all of the same sign adjacent one major surface of the layer being opposite an approximately equal number of subsurface charges bearing the opposite sign located adjacent the other remaining major surface of the layer. The subsurface charges of the electret may be distributed either uniformly along the length of the layer or separated by unpolarized strips of material in a pattern which will breakeup an image projected onto the surface of the layer such as the fine line arrangement previously recited. The general charge distribution described for the electret layer is easily obtained with polar dielectric materials whose molecules are permitted to move in an electric field whereupon the positively charged ends of the molecules become aligned adjacent one major surface with the negatively charged ends becoming aligned adjacent the opposite major sur- Satisfactory electret materials for practice of the present recording process comprise deformable dielectric substances which possess a finite dipole moment in the liquid or softened state including such natural organic products as carnauba wax, carnauba wax-beeS-wax mixtures, mixtures of carnauba wax with various synthetic thermoplastic polar resins, such as polyvinylchloride, and polar thermoplastic materials exclusive of additives including selected vinyls, acetals, acrylics, polyesters, and silicones. For reasons more fully amplified later in the specification, one preferred class of electret materials comprises polar thermoplastic compositions which also exhibit photoconductivity. Useful compositions in the latter class of materials include such diverse products as thermoplastic photoconductive polymers, mixtures of inorganic photoconductive solids with thermoplastic nonphotoconductive polymers, and mixtures of organic photoconductors with thermoplastic nonphotoconductive polymers.

In FIGURE 1 a recording member 1 of the invention is shown which comprises thermoplastic electret layer 2 supported on a substrate 3 of thermally-durable material that retains its physical form at the moderately elevated temperatures employed as recording. The base layer may be constructed understandably from a wide variety "of solid materials exhibiting heat stability in the 75-250 C. temperature range whereat the electret materials contemplated are softened which may include electrically conducting substances as well as dielectrics. As will be described in greater particularity hereinafter in the specification, selection of an electrically conducting base material also having resistive characteristics, such as metals or metal coated dielectrics, permits heating of the electret layer by electrical means using the base layer as a heating electrode. An electrically conducting base layer is not required as a ground plane during selective neutralization of the electret layer, however, since all electrical modification of the electret during recording takes place within the electret layer. Still, polarization of the electret layer prior to recording is facilitated with an electrically conducting substrate for the electret to serve again as a base electrode when applying a unidirectional field across the layer. From all these considerations, it will be noted that whereas the base layer is essential only as a physical support for the electret layer and this function is adequately provided with diverse nonconduoting materials, such as paper, ceramics, and plastics having a higher deformation temperature than the electret material, certain other advantages may be derived with electrically conducting base materials. It is also apparent that a base support for the electret layer comprising a nonconducting layer clad with a conducting film for contact with the electret layer is contemplated.

The uniform charge pattern 4 of electret layer 2 before recording is schematically represented in FIGURE 1 for better, visualization of the present recording process. Selective neutralization of the uniform polarization pattern under the hereinafter described recording conditions cancels positive and/ or negative charges 'in the affected portions of the electret material to produce a residual latent image charge pattern of the information being recorded. The latent image charge pattern in the electret layer consists of uncharged or neutralized cross-sectional segments of electret material bounded by still polarized material. Retention of residual polarization in the electret layer is of long-term duration and permits editing of a recording to add further information on the yet unrecorded portions of the layer at some future time. Heat development of the selectively discharged pattern in the electret layer produces surface deformations 5 having the general configuration depicted in FIGURE 1.

A suitable electret layer for recording may be prepared by placing heating electrodes in contact with both major surfaces of a useful thermoplastic film, heating the thermoplastic material to a liquid condition and allowing the liquid thermoplastic material to solidify in a strong unidirectional field. The electret layer may thereafter be adhered to any suitable support layer to provide the final recording member after removal of the electrodes. Preparation of the final recording member is facilitated if the thermoplastic film for the electret layer is first provided with a conducting backing .to serve both as an electrode during electret formation and as the final support element of the recording member. With this type construction only one additional electrode is needed for electret formation with the final recording member being provided by stripping the added electrode from the solidified electret layer. An alternate charging procedure may be used to form the electret wherein charge distribution is oriented in a fine line pattern of spaced polarized strips separated by corresponding unpolarized strips for dissection of the image being recorded, if gray scale in the final deformation image is desired. With this procedure, no further means need be employed during the recording process to dissect the recording image. If the electret layer being prepared is also photoconductive, premature light exposure of the charged layer is to -be avoided understandably to prevent accidential deponeutralizes or cancels the uniform charge in the electret layerresponsive to a pattern of electrons discharged from an energized cathode ray tube onto the electret layer. Charge neutralization may be accomplished in this manner on recording member 1 having a volume-polarized electret layer 2 supported on base layer 3 by simply placing the tube face in confronting approximate adjacency to the electret layer so that the discharged electrons will be physically transferred to the electret. The provision of a potential gradient between the particular electron discharge device being employed and the electret surface is a direct and convenient method of achieving the charge transfer. Other apparatus means for providing elec-. trons selectively in order to neutralize positive charges in the electret layer are contemplated including known devices such as an electron beam, an electrified probe, and the like. For charge neutralization, the faceplate of a cathode ray tube 6 having a plurality of spaced-apart electrified wires 7 extending therethr-ough is placed in confronting adjacency with the exposed electret surface. The tube is connected in a conventional electrical circuit (not shown) for electrifying the faceplate wires selectively in accordance with aninformation signal received by the tube. In this manner of operation, there is provided an image pattern of electrons on the exterior faceplate surface of the tube. A potential gradient existing between the charged wires and the electret layer allows the electrons to be physically transferred across the gap between the faceplate and the electret surface thereby achieving selective neutralization of the electret charge. Thus, it is not necessary for charge transfer that the confronting surfaces be in physical contact with the amount of spacing that can be tolerated being a function of the electrical potential available in the charged wires to effect transfer. Retention of a physical gap between the surfaces permits heat-development of the residual charge pattern in the electret layer at the same location employed for the neutralizing step. Although physical separation of these members can always be made before heat development of the final image, an additional process step is avoided by the contemplated recording procedure. With selective neutralization of positive charges 4 in the electret layer by the procedure described, there is obtained a latent image of depolarized electret material surrounded by the originally polarized medium. The latent image 8 depicted in FIGURE 2 comprises a grid pattern of bar-s for simplicity of illustration although it will be apparent that other type images, such as photographic scenes, portraits, and the like, may also be recorded in the manner described.

Development of deformations in the selectively depolarized recording member 1 may be accomplished by simple heating of the electret layer as shown in FIGURE 2 with a source of infrared radiation emanating from lamp means 9. Again it will be obvious that equivalent heating means may be substituted for infrared radiation to soften the thermoplastic electret material sufficiently for the residual polarization therein to selectively deform the exposed surface of the layer. It will also be re membered that employment of a base layer in the recording member having electrically resistive characteristics permit electric heating either by contact or induction heating techniques. Heating of the electret material occurs with improved efficiency in the modified procedure by heat conduction through the contacting support layer thereby eliminating many of the heat losses experienced with radiation type heating. Contact of a conducting metal support layer with heating electrodes (not shown) provides a. satisfactory resistance heating arrangement for the development procedure. The deformation pattern 5 produced in the deformable electret layer by heating the electret material to a softened condition corresponds in a point-by-point relationship with the latent image pattern. As depicted in the drawing, a grid image is reproduced on the surface of the electret layer which appears as a series of corresponding crests and grooves in the surface. The deformations are produced by movement of electret material responsive to the residual internal polarization after the material viscosity has been reduced sufliciently by heating to remove the restraining force to movement. Reduction of viscosity in the electret material to a value around 4000* centipoises is deemed adequate for deformation to proceed under the influence of the internal polarization. Subsequent cooling of the deformed soft electret material preserves the deformations permanently unless deliberately erased by reheating for a long enough time period to allow complete randomization of the charge carrying molecules in the electret layer. Erasure is most advantageously conducted at higher temperatures than required for production of the deformations so that liquidfied material flows by gravity forces alone. Since no appreciable degradation of electret material occurs during the manner of heating described, an erased recording member may again be used for information storage after reproducing a uniform electret charge in the material.

In FIGURE 3 there is depicted another information storage system of the invention which utilizes a recording member having an electret layer that is also photoconductive. Employment of a photoconductive electret layer eliminates need for an external source of neutralizing charges during the recording process. As distinct from the recording system of the preceding embodiment, selective depolarization of the present electret may be achieved entirely with an impinging beam of modulated photoradiation to produce a latent image charge pattern in the recording layer. A latent image which can be developed by the same heating procedures previously recited is formed in the photoconductive electret layer directly responsive to the impinging radiation. The electret layer is selectively neutralized in those areas receiving the radiation due to a localized increase in electrical conductivity which allows the positive and/or negative charge carriers to migrate and neutralize the electret derived charges. Selectiv neutralization of the photoconductive electret layer to produce a heat-developable latent image occurs in direct response to impinging photoradiation with no need for associated electrical charging apparatus or auxiliary electrical circuits during the recording process.

In FIGURE 3 there is shown a simple photocopying system employing a deformable photoconductive electret recording member 10 in accordance with the above-recited principles. Illumination means 11 for exposing the electret layer to an image pattern of the photoradiation may be provided readily by locating an image bearing photographic negative 12 between the recording surface and a lamp source 13 of the radiation. A l-aten-t image 14 of information contained on the negative, which again comprises a grid pattern 15 for simplicity of illustration, is formed in the electret layer by selective depolarization resulting from the light exposure. Heat development means 16 may simply comprise contacting electrodes 17 and 18 attached to the conducting base of the recording member 10 and connected to a conventional power source 19. A suitable recording member 10 for the system described comprises a laminated construction of deformable photoconductive electret layer 20 supported on an electrically conducting metal layer 21 which exhibits electrically resistive characteristics.

In FIGURE 4 there is shown in perspective view a preferred flexible recording member 22 of the invention having a tape construction which comprises a first thin photoconductive electret layer 23 in contact with a secconducting layer 24, all supported on a third flexible base layer 25 of thermoplastic material having a more elevated deformation temperature than the electret material. The individual layers of the recording member are all adhered into a composite unitary structure and have been depicted in greatly exaggerated thickness for clarity of illustration although the relative thickness of each layer is roughly in proportion to remaining layers of the structure. A flexible recording tape having the requisite degree of flexibility for winding the tape around small diameter mandrels may be constructed with a photoconductive thermoplastic electret layer ranging from around 250 microns in thickness, a conducting metal inner layer ranging in thickness from about 0.001-01 micron thickness, and a backing or support layer of suitable thermoplastic polymer ranging in thickness between 25 microns to 4 mils or greater. Keeping the recording layer as thin as consistent with a continuous film improves the performance characteristics of recording since the internal polarization gradient across the film increases with decreasing film thickness. Definition of the recorded image improves with increasing polarization gradient in the electret layer. A recording tape of this construction may be prepared from a commercially available polyethylene terephthalate film by first vacuum depositing a thin transparent conducting metal film on the surface of the polymer film. may have advantageously an electrical resistance of around 1,000 to 10,000 ohms per square centimeter if the metal layer is to be employed as a heating electrode for the recording member. Transparent conducting film exhibiting this resistivity characteristic are known and may be prepared from chromium, iron, nickel, as well as such metallic compounds as indium oxide and cuprous iodide. A detailed description of a method and apparatus for producing such a cuprous iodide film is disclosed in US. Patent 2,756,165, entitled Electrically Conducting Films and Processes for Forming Same, by

D. A. Lyon, issued July 24, 195 6. The conducting layer of the preferred recording member is optically transparent to permit readout of the final information-bearing deformations by transmissive Schlieren optics, although reflective readout systems may also be used satisfactorily to retrieve the information from recording members having an optically opaque conducting layer. A thin surface film of the solid electret composition may be deposited on the conducting layer by conventional technique such as coating the surface with an organic liquid solution of the electret material and thereafter removing the solvent. It is preferred to employ a thermoplastic polymer electret material which remains a solid at ordinary ambient conditions and further exhibits low adhesion as a solid to the backing member of the tape. Other useful properties of a thermoplastic polymer for the electret layer include an electrical resistivity of approximately ohm-centimeters at ordinary temperatures, a viscosity of about 4,000 centipoises in the soft- -ened state, ability to be softened at temperatures of around 85 C. depending on the thermal stability of the substrate, and absence of cold flow at ordinary temperatures. It is obvious from the listed properties that mixtures of thermoplastic organic polymers may also be employed for the electret material. A photoconductive electret composition may be obtained from the preferred polymers simply by dispersing such known photoconductors as selenium, sulfur, organic dyes, and the like, in the polymer material. Alternately, known photoconductive thermoplastic polymers may be used as the electret material having the physical properties above described. By photoconductive as used in the specification and claims is meant electrical behavior of the final recording layer to exhibit a dark resistivity of approximately 10 ohm-centimeters with a corresponding reduction in resistivity for the medium upon illumination sufficient to permit selective charge cancellation in the medium. Preferred backing or substrate materials for the tape are thermoplastic polymers which have a higher deformation temperature than the electret material, provide an adherent substrate for the conducting layer, are

optically transparent for transmissive optical readout of stored information on the tape, and possess suflicient thermal durability so as not to lose mechanical strength at the softening temperature of the electret material.

From the foregoing description, it is apparent that an improved system and a method of recording information The deposited metal film directly on a deformable storage medium in the form of permanent physical deformations has been provided. Additionally, useful information storage members have been provided which may be developed to provide a relief image directly by heating the selectively polarized member. It will be apparent from the foregoing description that certain process steps now conventionally employed in electrophotography may also be employed to selectively depolarize the present recording members to produce the characteristic latent image charge pattern. Likewise, other energy means than 'hereinbefore specifically disclosed may be used to selectively neutralize a uniform electret charge, equivalent energy means including such diverse radiant energy forms as high energy ionizing radiation and X-rays, as well as more conventional type radiation. It is intended to limit the present invention, therefore, only to the scope of the following claims.

What I claim as new and desire to secure by Letters Patent of the Uni-ted States is: V

1. A method for recording information in the form of light-modifying deformations on the surface of a recording member which comprises selectively neutralizing a supported volume-polarized thermoplastic electret layer in an image pattern of the information being recorded, and heating the selectively neutralized electret layer to produce deformations corresponding to the image pattern.

2. A method for recording information in the form of light-modifying deformations on the surface of a recording member which comprises depositing electrons on a supported volume-polarized thermoplastic electret layer in an image pattern of the information being recorded whereby the electret layer is selectively neutralized in portions receiving the electrons, and heating the selectively neutralized electret layer to produce deformations corresponding to the image pattern. a

3. A method for recording information in the form of light-modifying deformations on the surface of a recording member which comprises selectively neutralizing a supported volume-polarized photoconductive thermoplastic electret layer by exposing the layer to pho-toradiation in an image pattern of the information being recorded, and heating the selectively depolarized electret layer to produce surface deformations on the layer corresponding to the image pattern.

4. A method for recording information in the form of light-modifying deformations on the surface of a recording member which comprises physically transferring electrons from an energized cathode ray tube in an image pattern to a supported volume polarized thermoplastic supported volume-polarized photoconductive thermoplastic electret layer by exposing the layer to photoradiation in an image pattern of the information being recorded, heating the selectively neutralized electret layer to a softened condition whereupon surface deformations are formed on the layer in response to forces produced by the residual charge pattern, and cooling the layer to a solid state to preserve the deformations.

6. A method for recording information in the form of light-modifying deformations on the surface of a recording member which comprises selectively neutralizing a supported volume-polarized thermoplastic electret layer in an image pattern of the information being recorded, the electret layer before recording having a charge distribution pattern of spaced polarized strips separated by corresponding unpolarized strips for dissection of the image being recorded, heating the selectively neutralized electret layer to a softened condition whereupon surface deformations are formed on the layer in response to forces produced :by the residual charge pattern, and cooling the layer to a solid state to preserve the deformations.

7. An information storage member for recording information in the form of light-modifying deformations on the surface of the member which comprises a first layer of thermoplastic volume-polarized photoconductive electret material, a second conducting layer, and a third dielectric support layer of solid thermally-durable material which retains its physical form at elevated temperatures whereat the electret material is in a softened condition.

8. An information storage member for recording information in the form of light-modifying deformations on the surface of the member which comprises a first layer of thermoplastic volume-polarized electret material, a second conducting layer, and a third dielectric support layer of solid thermally-durable material which retains its physical form at elevated temperatures whereat the electret material is in a softened condition.

9. An information storage member for recording information in the form of light-modifying deformations on the surface of the member which comprises a first layer 10 of thermoplastic volume-polarized photoconductive electret material supported on a second layer of solid thermally-durable material which retains its physical form at elevated temperatures whereat the electret material is in a softened condition.

References Cited by the Examiner UNITED STATES PATENTS 1,891,780 12/1932 Rutherford 179--100.1 2,698,928 1/1955 Pulvari 179-1001 2,775,650 12/1956 Mason 179100.1 2,985,866 5/1961 Norton 340173 3,000,735 9/1961 Gunning 961 3,055,006 9/1962 Dreyfoos 346---74 3,066,298 11/ 1962 McNaney 346-74 

1. A METHOD FOR RECORDING INFORMATION IN THE FORM OF LIGHT-MODIFYING DEFORMATIONS ON THE SURFACE OF A RECORDING MEMBER WHICH COMPRISES SELECTIVELY NEUTRALIZING A SUPPORTED VOLUME-POLARIZED THERMOPLASTIC ELECTRET LAYER IN AN IMAGE PATTERN OF THE INFORMATION BEING RECORDED, AND HEATING THE SELECTIVELY NEUTRALIZED ELECTRET LAYER TO PRODUCE DEFORMATIONS CORRESPONDING TO THE IMAGE PATTERN. 